As a conventional technology of this sort, there is the technology described in JP-A-2005-83223. This conventional technology features that in a moving vehicle equipped with an engine, a reducing agent, for example, an aqueous solution of urea or the like which readily gives off ammonia is injected and fed from a reducing agent tank, in which the reducing agent is stored, into an exhaust system of the engine to lower the concentration of nitrogen oxides NOx in exhaust gas. This patent document discloses a construction provided with a temperature detector for detecting a temperature of the aqueous solution of urea stored in the reducing agent tank, that is, an aqueous urea solution tank; an additional coolant recirculation passage branched out from a coolant recirculation passage for the engine and communicating into the aqueous urea solution tank; two three-way cocks, a coolant recirculation pump and a solenoid valve, all of which are arranged on the additional coolant recirculation passage; and a heat exchanger arranged in the aqueous urea solution tank.
When the temperature of the aqueous urea solution in the aqueous urea solution tank becomes high, a possibility may arise that ammonia may be given off. When the temperature of the aqueous urea solution conversely becomes low, another possibility may arise that freezing may take place to pose a problem for injecting and feeding the aqueous urea solution into the exhaust system of the engine. With the foregoing in view, the conventional technology constituted as mentioned above is designed such that, when the temperature of the aqueous urea solution in the aqueous urea solution tank as detected by the temperature detector has risen beyond a predetermined temperature range to such a high temperature as possibly giving off ammonia, coolant for the engine is guided into the above-mentioned additional coolant recirculation passage to lower the temperature of the aqueous urea solution by means of the heat exchanger in the aqueous urea solution tank and, when the temperature inside the aqueous urea solution tank is conversely lower than the predetermined temperature range and is at such a temperature as possibly causing freezing, the coolant for the engine is guided into the above-mentioned additional coolant recirculation passage to raise the temperature of the aqueous urea solution by means of the heat exchanger in the aqueous urea solution tank. In this manner, the temperature of the aqueous urea solution in the aqueous urea solution tank is maintained at a temperature within the predetermined temperature range that causes neither giving-off of ammonia nor a freezing-related problem.